Mueller reportedly interviewed Michael Cohen for hours about every aspect of Trump's dealings with Russia

Michael Cohen reportedly sat down for hours of interviews with the special counsel Robert Mueller that spanned several sessions last month.

Mueller asked Cohen about President Donald Trump's business dealings with Russia, potential collusion with Russia, and whether Trump or any of his associates had discussed a pardon with Cohen, ABC News reported Thursday.

Cohen is a key figure in multiple threads of the Russia investigation, and his lawyer, Lanny Davis, said earlier that given his role as one of the president's closest confidants, Cohen "knows almost everything about Mr. Trump."

Michael Cohen, President Donald Trump's former longtime lawyer and fixer, sat down with the special counsel Robert Mueller for hours of interviews spanning multiple sessions over the last month, ABC News reported Thursday.

Mueller is said to have asked Cohen about every aspect of Trump's dealings — financial, political, and otherwise — with Russian interests.

The special counsel is tasked with investigating Russia's interference in the 2016 US election and whether members of the Trump campaign colluded with Moscow to tilt the race in his favor. Cohen was the subject of a separate investigation by the Manhattan US attorney's office into whether he committed financial crimes and campaign finance violations connected to two payments made shortly before the election to women who claim to have had affairs with Trump.

Cohen pleaded guilty last month to eight counts of tax evasion, one count of bank fraud, and two counts related to campaign-finance violations. He is now cooperating with that investigation, as well as a separate New York state investigation into the Trump Organization.

But his sit-down with Mueller was entirely voluntary, sources told ABC News, and did not include any promise of leniency on the part of prosecutors.

In addition to discussing Trump's business dealings and potential collusion with Russia, Mueller's team also reportedly asked Cohen whether Trump or any of his associates discussed the possibility of a pardon with Cohen.

That line of questioning would suggest the special counsel is continuing to gather new information as part of a parallel investigation into whether Trump sought to obstruct justice after the existence of the Russia investigation became public knowledge last year.

What does Cohen know?

Cohen is a key figure in several threads of the Russia investigation, including the creation of a Russia-friendly "peace plan" during the early days of Trump's presidency, as well as an allegation that Cohen traveled to Prague during the summer of 2016 to meet with Kremlin-linked officials.

Cohen was also central to the Trump Organization's push to build a Trump Tower in Moscow at the height of the campaign.

Felix Sater, the Russian-born businessman and Trump associate who worked with Cohen to push the deal, confirmed earlier this year that the Trump Organization was actively negotiating with a sanctioned Russian bank to secure financing for the deal at the height of the election.

Last month, it also emerged that Cohen had said Trump knew in advance about a Russian lawyer's offer to the campaign of "dirt" on the Democratic nominee Hillary Clinton in 2016. Cohen is said to have claimed that he was one of the people in the room when Trump greenlit the meeting, which is one of the focal points of the Russia probe.

Cohen's lawyer, Lanny Davis, later walked back that claim, however, saying he could not independently confirm it.

When news first broke in April that FBI agents had raided Cohen's property as part of their investigation into him, Trump defended him and slammed the raid as a "disgraceful situation."

"It's a total witch hunt," he said at the time, adding that "attorney=client privilege is dead."

But when it emerged that Cohen had pleaded guilty, the president turned on his former lawyer and excoriated him for striking a deal with prosecutors.

It's unclear how much useful information Cohen had for the special counsel.

But Davis suggested last month that Cohen could shed light on quite a bit.

"Mr. Cohen has knowledge on certain subjects that should be of interest to the special counsel, and is more than happy to tell the special counsel all that he knows," Davis said during an interview with MSNBC's Rachel Maddow.

In a later interview on CNN, Davis suggested Cohen's tenure in Trump's inner circle made him privy to details that have yet to be revealed to Mueller and the broader Russia probe.

"Michael Cohen, being a lawyer for Donald Trump for many, many years, knows almost everything about Mr. Trump," Davis said.